Sediment suspension and bed morphology in a mean shear free turbulent boundary layer

Johnson, Blair; Cowen, Edwin A.

doi:10.1017/jfm.2020.222

Cited by 12 publications

(14 citation statements)

References 69 publications

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“…To identify the wall boundary, the data were divided into segments. There is negligible change in the inner boundary within 50 s (Johnson & Cowen 2020); thus the data were divided into several 50 s (200 image pairs) intervals for subsequent processing. The brightness at the wall boundary is always strong.…”

Section: Experimental Set-up and Methodologymentioning

confidence: 99%

Experimental investigation of the effects of particle near-wall motions on turbulence statistics in particle-laden flows

2022

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Experiments on particle-free and particle-laden flows with the same incoming velocity were conducted in a horizontal wind tunnel. Three cases of particle-laden flows with different degrees of particle near-wall motions and similar particle volume fractions, including top-release particles, local-laying sand beds and global-laying sand beds, were designed to investigate the effects of collision bounces and impact splashing on turbulence statistics. The top-released particles accelerate the fluid during gravitational settling, but weaken the intensity and reduce the probability of ‘ejection’ and ‘sweep’ events. This leads to a weakened Reynolds stress and a decreased scale of the outer spectral peak at the centre of the logarithmic region, indicating a concentration of energy at small scales. In contrast, the collision bounce and impact splashing slow the fluid but promote the ‘ejection and sweep’ cycle with larger intensity, and thus enhance the Reynolds stress. Meanwhile, the bouncing and splashing generate ascending particles that transport the kinetic energy upwards, resulting in more energetic very-large-scale motions further from the wall. This study reveals the importance of particle motions to turbulence, and contributes to a further insight into the interactions between particles and turbulence in two-phase flows with erodible surfaces.

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Section: Experimental Set-up and Methodologymentioning

confidence: 99%

Experimental investigation of the effects of particle near-wall motions on turbulence statistics in particle-laden flows

2022

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“…They show that an oscillating plate in the water above a fluid-saturated particle bed induces pressure gradients that provoke particle motion and a formation of a heap. This is an example of failure in absence of mean shearing [2], in which the formation of a heap is associated with creeping that causes small rearrangements of the particles, likely driven by a gradient in pressure rather than shear stresses [3][4][5]. Here, we extend the experimental activity by employing plates with different dimensions and flexibility.…”

Section: Introductionmentioning

confidence: 93%

Particles bed morphology under an oscillating stiff and flexible plates

et al. 2021

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We analyse the different morphologies induced by an oscillating plate above an erodible bed. We present some data describing how the shape and the stiffness of the plate affects the main features of the generated heap. We investigate several configurations with different geometries, frequencies and the amplitudes. Some preliminary results are available, in which the role of the flexibility of the plate is taken in account. Unlike the rigid plate in which a proper oscillation induces the formation of one heap, the morphology of the bed is now characterized by more than one heap due to a different pressure profile induced by a flexible plate.

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“…The inability of the models to resolve the sub-grid turbulence, may result in only a subset of resuspension events being simulated, and those resulting from mean shear-free boundary turbulence (Johnson and Cowen, 2020) being neglected (e.g., HFIW events). Thus, we applied Eq.…”

Section: Turbulence-based Parameterizations In Rans Modelsmentioning

confidence: 99%

“…The 𝜏 defined by existing threshold models is most often determined by flume experiments using mean current velocity profiles (Shields, 1936;Soulsby et al, 1997). However, on larger scales and in more complex systems (e.g., shallow marine environments and large lakes), the threshold could be reduced because of the enhanced intensity of intermittent turbulent events (Salim et al, 2018;Yang et al, 2016), including resuspension from mean shear-free turbulence (Johnson and Cowen, 2020). Therefore, parameterizing 𝜏 from time-averaged current speeds is not always appropriate for modelling the bottom nephyloid layers in the presence of trubulent bursting events (Bourgault et al, 2014;Aghsaee and Boegman, 2015), including the turbid hypolimnion beneath HFIWs in this study.…”

Section: Parameterization Based On Near-bed Turbulencementioning

confidence: 99%

Observation and parameterization of bottom shear stress and sediment resuspension in a large shallow lake

Lin

Boegman

Jabbari

et al. 2022

Preprint

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Parameterizations for bottom shear stress are required to predict sediment resuspension from field observations and within numerical models that do not resolve flow within the viscous sublayer. This study assessed three observation-based bottom shear stress (τb) parameterizations, including (1) the sum of surface wave stress and mean current (quadratic) stress (τb= τw +τc); (2) the log-law (τb= τL); and (3) the turbulent kinetic energy (τb= τTKE); using two years of observations from a large shallow lake. For this system, the parameterization τb= τw +τc was sufficient to qualitatively predict resuspension, since bottom currents and surface wave orbitals were the two major processes found to resuspend bottom sediments. However, the τL and τTKE parameterizations also captured the development of a nepheloid layer within the hypolimnion associated with high-frequency internal waves. Reynolds-averaged Navier-Stokes (RANS) equation models parameterize τb as the summation of modeled current-induced bottom stress (τc,m) and modelled surface wave-induced bottom stress (τw,m). The performance of different parameterizations for τc,m and τw,m in RANS models was assessed against the observations. The optimal parameterizations yielded root-mean-square errors of 0.031 and 0.025 Pa, respectively, when τc,m, and τw,m were set using a constant canonical drag coefficient. A RANS-based τL parameterization was developed; however, the grid-averaged modelled dissipation did not always match local observations, leading to O(10) errors in prediction of bottom stress. Turbulence-based parameterizations should be further developed for application to flows with mean shear-free boundary turbulence.

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Sediment suspension and bed morphology in a mean shear free turbulent boundary layer

Cited by 12 publications

References 69 publications

Experimental investigation of the effects of particle near-wall motions on turbulence statistics in particle-laden flows

Experimental investigation of the effects of particle near-wall motions on turbulence statistics in particle-laden flows

Particles bed morphology under an oscillating stiff and flexible plates

Observation and parameterization of bottom shear stress and sediment resuspension in a large shallow lake

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